The present invention relates generally to a device (of the type specified in the description of claim 1) for measuring the contour of a surface along a scanning line. Thus, such measuring devices include a transducer with a scanning arm. The scanning arm includes a sensing tip on one end and is pivotally connected to an advance mechanism on the other end. The transducer emits an electrical signal related to the excursion of the sensing tip transducer to a scanning line. The transducer is interconnected to a recording device that records the electrical signal on the basis of distance or time. Such devices may include a sensing tip on the end of a scanning arm. Such devices are well known. The devices have the disadvantage that the sensing tip makes movements not only vertical to the scanning line, but also in the direction of the scanning line in relation to the said pivot bearing of the scanning arm. Since a commonly used recording device, e.g., a paper recording instrument, records at a constant speed, errors occur due to the movements of the scanning tip in relation to the pivot bearing of the scanning arm. In practice, such movement leads to an advance or lag of the scanning tip with respect to a constant advance and thereby to errors in the representation of the contour of the measured surface. Although these errors are smaller the longer the scanning arm is, the scanning arm, for reasons of stability, weight, and vibration strength, cannot be made to just any length.
Electro-mechanical measuring transformers are known in which the sensing tip is on a measuring pushrod that is held in a parallel guide, such as, for example, a ballbearing guide. The mass of such a pushrod, however, is relatively large, so that it is not suitable for rapid scanning and, in particular, is not suitable for accurately scanning the structure of a fine surface.
DE-OS No. 31 52 731 discloses a device for measuring the contour of a surface along a scanning line (according to the descriptive part of claim 1) which is supposed to solve the problem of avoiding measuring errors caused by excursion errors of the scanning arm in the scanning direction. The same is true for DE-PS No. 26 11 476. In the device disclosed in DE-OS No. 31 52 731, the scanning arm pivots with respect to the scanning arm support. The pivot joint of the scanning arm is made movable in a direction that is the advance direction of the arm support. Also provided is a connecting member. One end of the connecting member is supported pivotably by the arm. The other end is pivotably supported by a support on the scanning arm support on the one side or on the opposite side of the surface of revolution of the scanning arm. In the device disclosed as DE-PS No. 26 11 476, a first and a third rod are each connected pivotably with a crosshead guide which moves the scanning arm. A second rod is pivotably connected to the free end of the first and third rods. The scanning arm is fastened to the second rod.
In both these devices, there is thus a compensation of the circular motion of the scanning point on the scanning arm by means of a mechanical compensation. Such mechanical compensation is expensive. Moreover, the burden of the weight increases the inertia of the scanning arm and thus permits only a considerably lower scanning speed.